Ora-12521, listener does not currently know of instance requested in connect descriptor (4248511)

Ora-12521, listener does not currently know of instance requested in connect descriptor

Cannot get Toad to recognize or resolve the entries in my tnsnames.ora file. Trying to connect to an Oracle DB via VPN connection, and for one instance, can connect, but for another cannot.

 

The listener received a request to establish a connection to a database or other service. The connect descriptor received by the listener specified in addition to the service name an instance name for an instance (usually a database instance) that either has not yet dynamically registered with the listener or has not been statically configured for the listener. This may be a temporary condition such as after the listener has started, but before the database instance has registered with the listener.
 

Wait a moment and try to connect a second time.

- Check which instances are currently known by the listener by executing: lsnrctl services <listener name>

- Check that the INSTANCE_NAME parameter in the connect descriptor specifies an instance name known by the listener.

- Check for an event in the listener.log file.

- Check with DBA to confirm tnsnames.ora entry and info in the Listener.ora are in sync.